Process for removing radioactive impurities from gases



April 1965 w. BALDUS ETAL PROCESS FOR REMOVING RADIOACTIVE IMPURITIESFROM CASES Filed Nov. 27, 1959 tlilll 5 Ill l-||| |||l| United StatesPatent 3,179,487 PRGCESES EUR REMOVING RADEGACTHVE IMPURITKES FRUM GAEWolfgang Baldus and Wilhelm Lchmer, Munich, Germany, assignors toGeselischaft fiir Lindes Eismaschincn Alrtiengeseilschatt, Munich,Germany, a German company Filed Nov. 27, 1959, Ser. No. 855,652

Claims priority, application Germany, Dec. 2, 1958,

3 Claims. (Cl. 23-2) This invention relates to the removal ofradioactive components, especially gaseous radioactive components, fromgaseous mixtures which may be circulating, such as found for instance inthe form of cooling and/ or protective gases in atomic piles or otherenclosed spaces badly contaminated with radioactivity.

Methods are known by which such impurities can be removed, for instanceby the use of adsorbers or other conventional purification processes,especially washing processes and the like. It has also been suggestedthat such a purification process be carried out in several stages, inthe same way that other purifications of gases and liquids with respectto non-radioactive substances are carried out in several stages.

In the case of radioactive impurities, however, which is dealt with inthis invention, special conditions prevail. For here a new problemarises, which is that when the purification process is finished thesubstances used for purification-be it washing substances, adsorption orabsorption agents or even purely mechanical or electrical filters-afterhaving taken up the radioactive substances are themselves polluted withradioactivity and must, therefore, in their turn, be rendered harmlessto their environment by some means.

The invention relates especially to a type of purification by use of anadsorption process, for instance by means of an adsorptive gel, activecharcoal, microfilters and/or the like. On the other hand, the scope ofthe invention extends beyond this particular embodiment.

The principal object of the invention is to insure that during executionof the purification process the purification agents which have becomeradioactive can subsequently be rendered harmless as simply andthoroughly as pos sible.

According to the invention, this object is achieved by carrying out thepurification by stages in such a manner that at least the major partandpreferably all--of the radioactive components, and most convenientlyonly the radioactive components, of the gas mixture, accumulate in asingle purification stage and thus at the end in a single purifyingagent, in particular an adsorption agent, whereas the othernon-radioactive impurities or components to be eliminated for otherreasons are retained in one or more other purification stages in otherpurifying agents.

The advantage of this solution of the problem is that the radioactiveimpurities are collected into the smallest possible space, because anadsorber need only be relatively small if it is to adsorb theradioactive-and not, also, the non-radioactiveimpurities or components.Such small structural units can be more easily decontaminated than canlarger ones. It has been suggested, for instance, that adsorbers chargedwith radioactive substances should be left inside the atomic pileunit-which latter is of course shielded from the outside-until theradioactive substances Shall have broken down into harmless or lessharmful decomposition products, after which the adsorbers can theneither be regenerated or their filling material can be taken out andremoved. It is also easier to handle a small contaminated structuralelement than a larger one;

thus, in the case of larger structural units, their transportation outof the zone of operation would be made considerably more difiicult bytheir larger dimensions.

The process according to the invention may conveniently be carried outby oxidizing the oxidizable components of the initially-used gas, forinstance, hydrogen and carbon monoxide, to substances which arerelatively easily congealed by cooling.

According to a special feature of the invention, the system for carryingout the process comprises a final purification stage in the form of anadsorber cooled to -70 C., which final adsorber is preceded by at leastone pre-dryer, especially a gel-adsorber, cooled to a temperaturebetween 0 C. and 70 C. The pre-dryer may with advantage be preceded by astage operating above 0 C., for instance at room temperature, for theelimination of components, especially organic components, which can becondensed at higher temperatures. This stage can in turn be preceded bya high-temperature stage in which the substances which are difficult toremove otherwise-for instance, the aforesaid substances, hydrogen andcarbon monoxide-are converted into water and carbon dioxide, forinstance by the use of a catalyst or by some other chemical process.

It is, however, also Within the scope of the invention that yet othertransformations should follow, and that altogether different stages andtypes of purification be chosen, in each case with the object ofsecuring the elimination of the radioactive substance or substances ascompletely as possible and the segregation of the radioactive substancein one stage and in one purifying agent. In principle this could even beachieved in a first step, which possibility has the advantage that theradioactive sub stances would not have to pass through, and hence, insome Way contaminate, any of the further purification stages. From theinvestigations made up to now, however, it appears more expedient toprovide for the elimination of the radioactive substances in the laststage, because the radioactive substances Which accumulate in atomicpiles, especially when the fuel elements of the same are not shielded,are often less easily removed than are the non-radioactive substancesand hence only their removal in the last stage can insure, at any rateup to a certain point, that yet other, non-radioactive substances arenot removed at the same time, which circumstance would, as alreadymentioned, involve an increase in the necessary adsorption material orthe like.

Another feature of the invention is that-all regeneration within thepurification system being avoidedthe cooling gas circuit is equippedwith separation means, in particular adsonbers, which retain at leastthe harmful components and are preferably all of the same type, whichcan be switched into the circuit successively and separately and are ofsuch a size and in such number that the sum of the working-times of allthe separation means, in particular adsorbers, is of about the sameorder of magnitude as is the lift of the atomic pile per se.

Thus the inventive concept consists in providing Within the atomic pileunit a series of separation means, in particular adsorbers, none ofwhich is regenerated, but which are switched off successively as soon asthey are full, and then remain in the pile unit until the radioactivitythey had assumed has diminished to a tolerable or acceptable degree sothat they can be dismantled and taken out of the atomic pile unit inorder either to be destroyed or to be overhauled by removal of theadsorptive agent or by means of a regeneration process. On the otherhand, after this delay, regeneration inside the pile may also beconsidered, for which the now less dangerous, or harmless, componentsmay be withdrawn from the pile through the usual type of conduit.

amass? about 0.5% to about 5% of said total stream of cooling gas. Thediverted cooling gas mixture, especially heliumneon mixture, reed fromthe impurities, leaves the purification system at B, to be returned tothe cooling gas circuit. The gas mixture to be purified is shown by asolidline; the purified mixture is shown by the broken line, while acold air stream required for the purification process is shown by adash-and-dot line. The latter enters the purification system at C in theform of liquid air and leaves it again at D, according to the invention,in the form of gaseous air generally at a temperature above C.

The whole system consists of three successive, connected parts, eachoperating in a different temperatur range: a high-temperature part, amedium-temperature or pre-cooling part and a low-temperature part, inwhich the actual separation, e.g., adsorption, of the harmful activecomponents takes place.

1 represents a catalyst unit in which any hydrogen contained in the gasmixture to be purified is oxidized to H 0 and any carbon monoxidecontained therein is oxidized to C0 2 represents a heating conduit forheating the catalyst l to a suflifiiciently high temperature. In heatexchangers 3, 4 and 5 the gas to be purified is cooled by the purifiedgas flowing in the opposite direction, after which the gas to bepurified is further cooled by means of cold air in the heat exchangers 4and 5. Between the heat exchangers 5 and 6 is a separator '7 for easilycondensed substances, especially water. 8, 8 and 8 represent three geldryers for use in succession, mainly for the separation of water vaporand carbonic acid. The temperature of approximately 60 C. required forthis operation is produced by means of a refrigerant of the same type asthat designated by the commercial name Freon, which refrigerant passesthrough conduits 9, 9 and 9", respectively, shown as dotted lines, insuch a way that through the chosen supply line for the refrigerant thereis no possibility that the refrigerant will become mixed with the gas tobe purified, even in the event of a leak.

Further refrigeration takes place in heat exchangers 10 and 11. Thenecessary cold therefor is produced by an evaporator 12 in which liquidair, which enters at C, is stored and simultaneously the purified gasflowing back is cooled.

13, 13' and 13" represent adsorbers filled with activated charcoal,which carry out the actual purification of the helium-neon mixture withrespect to the active components, the adsorbers 13, 13' and 13 beingplaced in use, one after the otherin the same way as are the dryers 9,9' and 9"-one being used until it has become saturated with impurities.Each adsorber is designed to operate for about 5,000 hours.concentrations met with in this case, this working life can be obtainedwith small-size adsorbers especially if appropriate non-activesubstances are added to the active material to be eliminated inparticular crypton and xenon, in the manner already suggested. When itsworking time has run out and the adsorber is fully charged withimpurities, the major part of the helium-neon mixture is pumped out ofit so that the adsorber can be warmed and can then remain in the storagechamber of the pile In view of the very small at any desiredtemperature. The working life of the adsorbers 13 and the gel-dryers 9as well as the liquid air unit required for the liquid air supply mayexpeiently be adjusted in relation to one another so that thereplacement of dryers and adsorbers and the thawing of the liquid airunit can always be carried out simultaneously.

The transfer of cold is effected by indirect heat-exchange in suchmanner that during heat-exchange between the purified and the as yetunpurified gas, the purified gas cts at the same time as protective gasfor the unpurified gas. The purified gas is also used as safety gas forthe valves and conduits, in each case being made to how through a jacketsurrounding the stream of unpurified gas.

In the purification system shown in the diagram, the arrangement is suchthat the purification from traces of water takes placeafter thecountercurrent coolers 6 and r-in the gel-dryers 8, so that no waterreaches the adsorbers 15. On the other hand, the radioactive inert gasesto be eliminated by them are practically entirely retained in thecharcoal adsorbers 13. In the case of the embodiment of the inventionshown as an example some CO does enter the charcoal adsorber 13;however, the quantity of charcoal which must be provided for the CO issmall in comparison with the quantity-incidentally, also not a verylarge amountrequired for the adsorption of the inert gases. 7

We claim:

1. Process for purifying a gas mixture containing radioactive impuritiesand hydrogen and carbon monoxide as nonradioactive impurities, whichcomprises oxidizing only the non-radioactive impurities of said mixturewith the aid of a catalyst mass to convert the hydrogen and carbondioxide to water and carbon dioxide, separating the water and carbondioxide from the gas mixture by condensation and adsorption at atemperature within the range 0 C. to -70 C., and separating theradioactive impurities from the gas mixture by adsorption in anadsorption agent at a temperature below 70 C.

2. Process defined in claim 1, in which the adsorption agent iscontained in a plurality of adsorbers which are each successivelyswitched into the gas purifying circuit until the end of their workingtime and the used-up adsorbers at the end of the periods required forthe removal of the radioactive impurities with which they are saturatedare removed from the circuit and replaced by active adsorbers.

3. Process for purifying a gas mixture circulating in an atomic pile,said gas mixture containing radioactive impurities and hydrogen andcarbon monoxide as nongas mixture by adsorption in an adsorption agentat a' temperature below -70 C., and leading the resulting purified gasmixture stream back to the atomic pile to circulate therein.

References Cited by the Examiner Hurst et al.: The Homogeneous AqueousReactor,

Nuclear Power, May 1957, pages 193-195.

Briggs and Swartout: Aqueous Homogeneous'iower Reactors, Proceedings ofthe International Conference on the Peaceful Uses of Atomic Energy,published by the United Nations, New York, 1955, vol. 3, pages 181185.

MAURICE A. BRINDISI, Primary Examiner.

1. PROCESS FOR PURIFYING A GAS MIXTURE CONTAINING RADIOACTIVE IMPURITIESAND HYDROGEN AND CARBON MONOXIDE AS NON-RADIOACTIVE IMPURITIES, WHICHCOMPRISES OXIDIZING ONLY THE NON-RADIOACTIVE IMPUITIES OF SAID MIXTUREWITH THE AID OF A CATALYST MASS TO CONVERT THE HYDROGEN AND CARBONDIOXIDE TO WATER AND CARBON DIOXIDE, SEPARATING THE WATER AND CARBONDIOXIDE FROM THE GAS MIXTURE BY CONDENSATION AND ADSORPTION AT ATEMPERATURE WITHIN THE RANGE 0*C. TO -70*C., AND SEPARATING THERADIOACTIVE IMPURITIES FROM THE GAS MIXTURE BY ADSORPTION IN ANADSORPTION AGENT AT A TEMPERATURE BELOW -70*C.